Method for feeding arsenic dopant into a silicon crystal growing apparatus

ABSTRACT

A feed assembly and method of use thereof of the present invention is used for the addition of a high pressure dopant such as arsenic into a silicon melt for CZ growth of semiconductor silicon crystals. The feed assembly includes a vessel-and-valve assembly for holding dopant, and a feed tube assembly, attached to the vessel-and-valve assembly for delivering dopant to a silicon melt. An actuator is connected to the feed tube assembly and a receiving tube for advancing and retracting the feed tube assembly to and from the surface of the silicon melt. A brake assembly is attached to the actuator and the receiving tube for restricting movement of the feed tube assembly and locking the feed tube assembly at a selected position.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/108,923, filed Apr. 24, 2008, and entitled, “METHOD AND APPARATUS FORFEEDING ARSENIC DOPANT INTO A SILICON CRYSTAL GROWING APPARATUS”, whichis hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention generally relates to the preparation ofsemiconductor grade silicon crystals, used in the manufacture ofelectronics. More particularly, the invention relates to a device forfeeding arsenic dopant into an apparatus for producing low resistivitysilicon crystals.

Silicon crystal growth using the Czochralski (CZ) method involveschanging the characteristics and properties of the silicon ingot beinggrown by adding a dopant material to the molten silicon before siliconingot growth. A common dopant material used in this process is arsenic.Arsenic, however, is a volatile substance and problems often arisethrough conventional methods of introducing the dopant to the siliconmelt.

One such method is to dump the dopant from a port positioned above themelt. However, because of the high temperatures of the process, there isa violent loss of arsenic to the argon gas environment above the melt.This results in the generation of oxide-particles which can prolong andcompromise the crystal growing process. Thus, this method is veryinefficient.

Another method uses a quartz vessel containing the dopant above the meltfor introducing the volatile gas to the melt. This method can reduceloss of vaporized dopant if the vessel has a port extending into themelt. Regardless, these methods result in complicated operation and lossof volatile dopant. The present invention overcomes these difficultiesand disadvantages associated with prior art processes by introducing thedopant to the melt at an upper surface of the melt.

SUMMARY OF THE INVENTION

In one aspect, the present invention includes a feed assembly forfeeding a dopant to a silicon melt in a crystal growing apparatus. Theassembly comprises a vessel for holding and releasing a dopant solidmaterial and an elongate feed tube operatively connected to the vessel.The feed tube comprises a fixed tube and a movable tube concentricallyarranged with the fixed tube. The assembly also includes an actuatorconnected to the moveable tube for moving the moveable tube relative tothe fixed tube for advancing the moveable tube toward an upper surfaceof the silicon melt in the apparatus and retracting the moveable tubeaway from the upper surface of the silicon melt to selectively positionthe moveable tube for introducing the dopant material released from thevessel to the silicon melt when the feed assembly is mounted on thecrystal growing apparatus.

In another aspect, the present invention includes a method for feedingarsenic dopant to a silicon melt in a silicon crystal growing apparatushaving a crystal growing chamber. The method includes placing granularsolid arsenic dopant in a vessel attached to a feed tube comprising afixed tube and a movable tube in concentric arrangement. The moveabletube is lowered toward the silicon melt with an actuator connected tothe moveable tube to selectively position the moveable tube at thesurface of the silicon melt. In addition, the dopant is released fromthe vessel to allow dopant to travel down the feed tube and into themelt at an upper surface of the melt.

In still another aspect, the present invention includes a method forfeeding arsenic dopant to a silicon melt in a silicon crystal growingapparatus having a crystal growing chamber. The method comprises placinggranular solid arsenic dopant in a vessel attached to a feed tubecomprising a fixed tube and a moveable quartz tube having an angled tip.The fixed tube and moveable quartz tube are in concentric arrangement.Further, the method includes lowering the moveable tube toward thesilicon melt with an actuator connected to the moveable tube toselectively position the moveable tube at the surface of the siliconmelt. Still further, the method comprises releasing the dopant from thevessel to allow the dopant to travel down the feed tube to a catchlocated in the moveable tube for catching the dopant material when it isreleased from the vessel. In addition, the method comprises introducingargon gas into the feed tube below the vessel causing sublimation of thedopant resulting in dopant laden argon exiting the angled tip of themoveable quartz tube at an upper surface of the silicon melt.

In yet another aspect, the present invention includes a feed assemblyfor feeding a dopant to a silicon melt in a crystal growing apparatus.The feed assembly comprises a vessel for holding and releasing a dopantsolid material and an elongate feed tube attached to the vessel. Thefeed tube includes a fixed tube and a movable tube concentricallyarranged with the fixed tube. Further, the feed assembly includes acatch located within the moveable tube for catching the dopant materialwhen it is released from the vessel.

Other objects and features will be in part apparent and in part pointedout hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a first embodiment of a feed assembly in aretracted position;

FIG. 2 is a front view of a vessel-and-valve assembly of the feedassembly with a portion broken away showing the flow of dopant material;

FIG. 3 is a cross section of a second embodiment of the feed assembly inan extended position;

FIG. 4 is a perspective of the feed assembly attached to a crystalgrower furnace chamber;

FIG. 5 is a perspective of the vessel-and-valve assembly and actuator ofthe feed assembly;

FIG. 6 is a perspective of an isolation valve of the feed assemblyattached to the crystal grower.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Multiple embodiments for an arsenic dopant feed assembly areillustrated. FIG. 1 illustrates a first embodiment of an arsenic dopantfeed assembly, generally designated by the reference number 10. In thefirst embodiment, the dopant feed assembly 10 is fabricated from arefractory material that is non-contaminating and non-reactive witharsenic, silicon and graphite.

The first embodiment of the feed assembly 10 comprises avessel-and-valve assembly 11 for holding dopant solid (not shown), and afeed tube assembly, generally indicated at 15, attached to thevessel-and-valve assembly 11 for delivering the dopant to a silicon melt(not shown). An actuator 19 is operatively connected between the feedtube assembly 15 and a receiving tube 21 for advancing and retractingthe feed tube assembly to and from an upper surface of the silicon melt.A brake assembly 25 is operatively connected between the actuator 19 andthe receiving tube 21 for restricting movement of the feed tube assembly15 and locking the feed tube assembly at a selected position. Anisolation valve 27 is provided at a bottom of the feed tube assembly 15.The valve 27 is configured for placing the feed assembly 10 incommunication with a crystal growing apparatus 31 (see FIG. 6).

Referring to FIG. 2, the vessel-and-valve assembly 11 includes a dopantcartridge 41 configured for holding the dopant solid and a valve 43attached to the feed tube assembly 15 that can be opened to release thedopant down the feed tube assembly. The valve 43 has a handle 45 foropening and closing the valve.

The feed tube assembly 15 comprises a series of elongate concentrictubes including a fixed tube 51 and one or more moveable tubes 53situated around the fixed tube and arranged in a telescoping fashion(see FIG. 1). The fixed tube 51 is closed at a first end 54 by a vacuumflange 55 and is received by the moveable tubes 53 at a second end 57(see FIG. 3). An end cap 59 at the first end 54 attaches the fixed tube51 to the receiving tube 21. The end cap 59 includes a seat 61 having anopening 63 which receives the first end 54 of the fixed tube 51. Anannular seal 65 seals the opening 63 between the end cap 59 and thefixed tube 51.

A vacuum fitting 67 connects each moveable tube 53 to an adjacentmoveable tube. Each vacuum fitting 67 includes two opposing ringfittings 69 connected to each other by a threaded coupling 71 engagingthreads 73 on the ring fittings. The embodiment illustrated in FIG. 1shows two moveable tubes, however a single moveable tube or three ormore moveable tubes are contemplated as being within the scope of thepresent invention.

The feed tube assembly 15 provides a passage 81 through which dopantmaterial travels when it is released from the vessel-and-valve assembly11. An outlet 83 of the moveable tubes 53 is in fluid communication withthe vessel-and-valve assembly 11 for introducing the dopant to thesilicon melt (see FIG. 3). In this first embodiment, the feed tubeassembly 15 can be made of any refractory material that isnon-contaminating and non-reactive with arsenic, silicon and graphite.As will be explained in greater detail later, a moveable tube 53′ of thesecond embodiment that is positioned at the surface of the melt isfabricated from quartz.

Referring to FIG. 1 the actuator 19 comprises a linear translator 85including an annular magnetic sleeve 87 attached to the moveable tubes53 and an annular magnetic slide 89 adjacent and magnetically coupled tothe sleeve. The magnetic sleeve 87 is sized and shaped for receiving themoveable tubes 53 in the sleeve. The sleeve 87 is secured to themoveable tubes 53 by friction fitting. The magnetic slide 89 is sizedand shaped for receiving the receiving tube 21 and directly engages theouter surface of the receiving tube 21. A small clearance 90 between thereceiving tube 21 and the magnetic slide 89 allows the magnetic slide toslide along the length of the receiving tube. The slide 89 is alignedwith the magnetic sleeve 87, creating a magnetic coupling due to theopposite polarization of the two structures. This coupling secures theslide 89 to the receiving tube 21 at the same height that the magneticsleeve 87 is positioned on the moveable tubes 53. As a result, movementof the slide 89 along the receiving tube 21 causes the magnetic sleeve87 to move under the force of magnetic attraction. As the slide 89 movesup and down the receiving tube 21, the moveable tubes 53 slide away fromand toward the fixed tube 51 for positioning a tip 91 of the moveabletubes 53 at the surface of the silicon melt (see FIG. 3). As will beexplained in greater detail below, the magnetic slide 89 also includesan extension 93 having an annular teardrop shape with an aperture 95 atits tapered end. The aperture 95 is configured for attaching to thebrake assembly 25. Although the preferred embodiment of the inventionincorporates the magnetically coupled linear translator, it isenvisioned that other suitable actuators (e.g., mechanical, electrical,or electromechanical) could be used without departing from the scope ofthis invention.

Referring to FIG. 4, the receiving tube 21 is an elongate tube made ofstainless steel. The receiving tube 21 separates a portion of theactuator 19 and feed tube assembly 15 from the surrounding environment(see FIG. 1). The feed assembly 10 is illustrated as having tworeceiving tube members 99 connected in series. However, any number ofreceiving tube members 99 is foreseen. A first seal assembly 101connects the receiving tubes 21. The seal assembly 101 comprises ano-ring 103 and a clamp 105 having semi-circular clamp halves 107. Asecond seal assembly 109 connects the receiving tube 21 to the isolationvalve 27. One clamp half 111 of the second seal assembly 109 has athreaded extension 113 for connecting the receiving tube 21 to theisolation valve 27 as will be explained in greater detail below.

Referring to FIGS. 1 and 5, the brake assembly 25 comprises stops 121,122 positioned on the receiving tube 21, a rod 123 (broadly, a “brakingmember”) disposed between the stops and a screw 125 (broadly, a lockingmember) engaging the braking member and the magnetic slide 89 forlocking the slide at a selected position along the receiving tube 21.Similar to the extension 93 on the magnetic slide 89, the stops 121, 122have an annular teardrop shape with a central opening 127 at its bulbousend for receiving the receiving tube 21 and a hole 129 at the taperedend extending from a top face 131 to a bottom face 133 for receiving thebraking member 123. A side face 135 on the tapered end has an adjustmentopening 137. The stops 121, 122 are positioned on the receiving tube 21above and below the magnetic slide 89. The braking member 123 passesthrough the hole 129 in the first stop 121, the aperture 95 in the slide89 and the hole 129 in the second stop 122. Thus, the braking member 123links the stops 121, 122 to the slide 89 creating a track 139 to guidethe slide along the receiving tube 21.

The stops 121, 122 are also adjustable. The central opening 127 is sizedand shaped for receiving the receiving tube 21. Similar to the magneticslide 89, a small clearance 140 between the stops 121, 122 and thereceiving tube 21 allow the stops to slide along the length of thereceiving tube. On the receiving tube 21 the stops 121, 122 can be slidto a selected position. Once the selected position for the stops 121,122 is achieved, a stop screw 141 can be inserted into the adjustmentbore 137 to lock the stops in place. The tip of the stop screw 141presses against the braking member 123 holding the stops 121, 122 inposition. The stop screw 141 can then be unscrewed to allow the stops121, 122 to move to another position on the receiving tube 21 andre-tightened to lock the stops in place again.

Referring to FIGS. 1 and 6, in one embodiment the isolation valve 27comprises a ball valve 151 having a body 153 and a passageway 155 with aball 157 disposed in the passageway mounted for selective rotationbetween open and closed positions (illustrated embodiment shown in openposition. A pair of valve seats 159, 161 are provided in the passageway155 on opposing sides of the ball 157. In the preferred embodiment, thevalve seats 159, 161 are located substantially equidistant from an axisof rotation of the ball 157 and include radial openings 163. The ball157 and valve seats 159, 161 are enclosed within the body 153 by a pairof end fittings 165. The end fittings 165 can be mounted to the body 153by any sufficient means. In the present invention, mounting bolts 167are utilized. At least one end fitting 165 is also provided withinternal threads 169 to facilitate connecting the isolation valve 27 tothe feed tube assembly 15 by the threaded extension 113 on the secondseal assembly 109. It is understood that any other convenient means ofconnecting the isolation valve to the feed tube assembly is within thescope of the present invention.

A stem assembly 171 and handle 173 are provided for actuating theisolation valve 27. The handle 173 is releasably secured to the stemassembly 171 by a nut 175 that clamps to the tip of a packing nut 177and also helps to support the ball 157 in the body 153. The ball 157 issupported in the passageway 155 such that the ball can shift axiallyalong the passageway. The ball valve 151 can be manually actuated withthe handle 173, or an actuator (not shown) may be provided to actuatethe valve. The positions of the handle 173 and the ball 157 are limitedby a depending catch member 179 carried by the handle. The catch member179 engages a surface of the body 153 to provide fixed stops for theisolation valve 27.

The structure of the isolation valve as described above reflects apreferred embodiment. It will be readily apparent to those skilled inthe art that changes and additions to the structure may be made toaccommodate specific operational requirements. Such modifications arenot deemed to affect the scope of the present invention.

Operation of this first embodiment of the feed assembly 10 is asfollows. Once the silicon melting process is complete, the actuator 19advances the moveable tubes 53 of the feed tube assembly 15 so theoutlet 83 of the moveable tubes 53 is located at the upper surface ofthe silicon melt. The locking member 125 of the brake assembly 25 istightened to lock the magnetic slide 89 in place, thus locking theoutlet 83 of the moveable tubes 53 in position at the surface of thesilicon melt. The dopant held in the vessel-and-valve assembly 11 isreleased when the valve 43 in the dopant cartridge 41 is opened. Thedopant will travel through the feed tube assembly 15, past an openedisolation valve 27 and into the silicon melt at the surface of the melt.The moveable tubes 53 are retracted by the actuator 19 and argon gas isreleased below the vessel-and-valve assembly 11 into the feed tubeassembly 15 for cooling the assembly 10. Finally, the assembly 10 isisolated from the crystal growing apparatus 31 by closing the isolationvalve 27.

As illustrated in FIG. 3, a second embodiment of the feed assembly 10′is designated in its entirety by the reference number 10′. Thecomponents of the second embodiment are exactly the same as the firstembodiment except for a modified feeding tube assembly 15′. The feedingtube assembly 15′ of the second embodiment comprises a moveable tube 53′made of a special quartz material. This material is used primarily toaccommodate gas phase doping. The quartz tube 53′ is a thick walledclear fused quartz tube with an outside diameter of about 25 mm, a wallthickness of about 3 mm and a length of about 711 mm. This tube 53′ hasan angled tip 91′ allowing a maximum melt surface area to be exposed todopant gasses flowing from the tip. Additionally, the moveable tube 53′includes a guide 201 aligning the quartz tube 53′ and a perforated disk203 preventing the dopant from exiting the tube 53′ into the siliconmelt 17. When the dopant material trapped in the tube 53′, argon gas canbe introduced into the feed tube assembly 15′ causing the dopant ladenargon to travel down the feed tube assembly 15′ under sublimation andexit the angled tip 91′ at the upper surface of the silicon melt.

This second embodiment of the feed assembly 10′ operates as follows. Theprocess is similar to the process described for the first embodimentexcept the actuator 19 advances the quartz tube 53′ of the feed tubeassembly 15′ so the angled tip 91′ is positioned at the upper surface ofthe silicon melt. The locking member 125 of the brake assembly 25 istightened to lock the magnetic slide 89 in position, thus locking theangled tip 91′ of moveable tubes 53′ in position at the upper surface ofthe silicon melt. The dopant held in the vessel-and-valve assembly 11 isreleased when the valve 43 in the dopant cartridge 41 is opened by thehandle 45. The dopant travels through the feed tube assembly 15′ and iscollected on the perforated internal disk 203. Argon gas is introducedinto the feed tube assembly 15′ below the vessel-and-valve assembly 11.Sublimation of the dopant will occur as the dopant is captured at theperforated disk 203. This process results in dopant laden argontraveling past the opened isolation valve 27 and out the angled tip 91′of the quartz tube 53′ at the upper surface of the silicon melt. Afterthe dopant has undergone sublimation, the quartz tube 53′ is retractedby the actuator 19 and the feed tube assembly 15′ is cooled with theflow of argon gas. Finally, the assembly 10′ is isolated from thecrystal growing apparatus 31 by closing the isolation valve 27.

Having described the invention in detail, it will be apparent thatmodifications and variations are possible without departing from thescope of the invention defined in the appended claims.

When introducing elements of the present invention or the preferredembodiments(s) thereof, the articles “a,” “an,” “the,” and “said” areintended to mean that there are one or more of the elements. The terms“comprising,” “including,” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above constructions and methodswithout departing from the scope of the invention, it is intended thatall matter contained in the above description and shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

1. A method for feeding arsenic dopant to a silicon melt in a siliconcrystal growing apparatus having a crystal growing chamber comprisingthe steps of: placing granular solid arsenic dopant in a vessel attachedto a feed tube, the feed tube comprising a fixed tube and a movable tubein concentric arrangement; lowering the moveable tube toward the siliconmelt with an actuator connected to the moveable tube to selectivelyposition the moveable tube at the upper surface of the silicon melt; andreleasing the dopant from the vessel to allow dopant to travel down thefeed tube and into the melt at the upper surface of the melt.
 2. Amethod of claim 1 further comprising the steps of retracting themoveable tube with the actuator and cooling the feed tube by introducingargon gas into the feed tube.
 3. A method of claim 2 further comprisingthe steps of isolating the feed tube from the growing apparatus with anisolation valve.
 4. A method of claim 1 wherein the moveable tube islowered by a linear translator magnetically coupled to the moveabletube.
 5. A method of claim 4 wherein a brake assembly attached to themoveable tube holds the moveable tube in a selected position forintroducing the dopant material at the upper surface of the siliconmelt.
 6. A method for feeding arsenic dopant to a silicon melt in asilicon crystal growing apparatus having a crystal growing chamber,comprising the steps of: placing granular solid arsenic dopant in avessel attached to a feed tube comprising a fixed tube and a moveablequartz tube having an angled tip, the fixed tube and moveable quartztube being in concentric arrangement; lowering the moveable tube towardthe silicon melt with an actuator connected to the moveable tube toselectively position the moveable tube at an upper surface of thesilicon melt; releasing the dopant from the vessel to allow the dopantto travel down the feed tube to a catch located in the moveable tube forcatching the dopant material when it is released from the vessel; andintroducing argon gas into the feed tube below the vessel causingsublimation of the dopant resulting in dopant laden argon exiting theangled tip of the moveable quartz tube at the upper surface of thesilicon melt.
 7. A method of claim 6 further comprising the steps of:retracting the moveable tube with the actuator; and cooling the feedtube with argon gas.
 8. A method of claim 7 further comprising the stepsof isolating the feed tube from the growing apparatus with an isolationvalve.
 9. A method of claim 6 wherein the moveable tube is lowered by alinear translator magnetically coupled to the moveable tube.
 10. Amethod of claim 9 wherein a brake assembly attached to the moveable tubeholds the moveable tube in a selected position for introducing thedopant material at the upper surface of the silicon melt.